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Part 2 ebook the present the content: ophthalmology for the medical examiner; ear, nose and throat (ENT) medicine and dentistry for aeromedical examiners; neurology for the aeromedical examiner; psychiatry; phychology; operational and clinical aviation psychology; neuropsychological disorders after brain injury; the man-man interface in the man-machine; passengers, passenger health.
Chapter 17 Ophthalmology for the Medical Examiner Rüdiger Schwartz*,† and Jörg Draeger† INTRODUCTION Over 90% of flight-relevant information comes to the pilot through the eye, making it the most important sensory input system for aviators Despite technological advances in aircraft engineering and information technology, nothing has changed in that regard On the contrary, research is being undertaken to transfer even more flight-relevant information from the aural into the visual realm With the introduction of “fly-by-wire” technology, even residual mechanical input is slipping away; neither stick pressure nor power setting changes are noticeable by feel or hearing when the Flight Management System (FMS) activates its programmed settings, whereas a glance at the primary FMS display provides the pilot with decisive information regarding the functional conditions and flight profile of the aircraft.1 Developments in recreational flying are heading in the same direction, with display panels becoming similar to those of corporate aircraft Even high performance airplanes, although not flown by an FMS, nonetheless have relatively large instruments displays that are not only designed for navigational purposes but are also linked to flight data recorder information The pilot can only utilize the displayed information if he can discern the information precisely * Corresponding author † Department of Ophthalmology, Hamburg University, Hamburg, Germany, E-mail: r.schwartz@yke.uni-hamburg.de 399 400 R Schwartz and J Draeger This chapter does not deal with the cognitive side of signal processing, but rather with the optical The eyes produce an unambiguous signal image, which is transmitted to the optical cortex for interpretation and reaction As in all of our sensory systems, a certain stimulus size must be exceeded in order to be perceived In physiology, this is termed “threshold.” This does not necessarily have to with energy levels, such as the quantum amount required to sense light, but rather with the resolution limits for distinguishing geometric patterns On the one hand, the cockpit signals must be designed such that they exceed the threshold for perception, and on the other hand, the receptor (the eye) and its performance must meet regulatory standards The concept of visual acuity encompasses many aspects and must therefore be narrowed down “Point visual acuity” is defined as the ability to discriminate a single point This can be important in aviation, such as when a distant aircraft can be discerned as only a point In clinical practice, however, point visual acuity is not measured quantitatively “Visual acuity of separation,” or minimum angle of resolution, describes the ability to discriminate two closely neighboring points as separate This is the conclusive criterion for signal perception “Visual acuity of localization” is defined as the smallest recognizable change in spatial relationships between two objects An example is the so-called “nonius” visual acuity, demonstrated by the ability to place one vertical line precisely on top of another line, such as is used in calipers with a vernier scale It is possible to determine inaccurate positions at less than 10 seconds of arc “Visual acuity of recognition” (minimum legible acuity): the point at which an object can be perceived as such This is within the realm of cognition The resolution of the eye is the visual acuity of separation as given by the smallest angle subtended by two points still visible as separate This angle is called the “minimum angle of resolution” (MAR) and is measured in minutes of arc The visus is defined as the reciprocal of this threshold angle, measured in minutes of arc Visus means the MAR is one minute, visus 0.5 means the MAR is two minutes, etc Sometimes, the log10 form of MAR is used Ophthalmology for the Medical Examiner 401 Relationship between Snellen notation, MAR, logMAR, and decimal notation: Snellen Notation Metric Imperial 6/60 6/48 6/38 6/30 6/24 6/19 6/15 6/12 6/9.5 6/7.5 6/6 6/4.8 6/3.8 6/3.0 20/200 20/160 20/125 20/100 20/80 20/60 20/50 20/40 20/30 20/25 20/20 20/16 20/12.5 20/10 MAR logMAR Decimal 10 8.0 6.3 5.0 4.0 3.2 2.5 2.0 1.6 1.25 1.00 0.80 0.63 0.50 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 −0.1 −0.2 −0.3 0.10 0.13 0.16 0.20 0.25 0.32 0.40 0.50 0.63 0.80 1.00 1.25 1.58 2.00 Many individuals, especially young people, possess a visus of >1 In other words, the MAR is significantly smaller than one minute Examination is usually performed using test letters with defined identifiable angles in the shape of physically defined figures, either Landolt coptotypes or numbers with defined angular relationships For maximal resolution, only an area of about 1° of the fovea centralis is needed If the image drifts onto the retinal periphery, the resolution will deteriorate exponentially At a distance of 20 cm from the eye, the area of clarity has a diameter of about 3.5 mm, while at a distance of a meter, this area enlarges to 17 mm It is therefore not possible to visualize two flight deck instruments simultaneously with full resolution, whether they are next to each other or at different distances To achieve this, eye movements are needed These are very fast and are directed with considerable accuracy, but this requires a distinctly structured space, in which particular features provide an optical orientation A display built from completely equal units will more likely lead to search errors than one where the units are marked 402 R Schwartz and J Draeger unequally This is particularly important for rapid reading of charts and checklists The MAR changes not only with the position of the image on the retina, but also with the lighting, due to the adaptability of the eye The eye adapts its sensitivity to changes in lighting conditions This occurs partly via regulation of the pupillary diameter and partly by adaptation of the sensitivity of the photoreceptors When it is bright, this occurs very rapidly by photoadaptation; this takes much longer in darkness when only the parafovea rods are being utilized scotoptically A quick glance from a bright instrument panel to a dimly lit region is not possible; dark adaptation takes much more time This is important at twilight or at night A similar situation exists when dealing with contrast, such as a figure on a display, when rapid and precise recognition is critical Precise object recognition is predicated upon optimal geometric optics projecting the corresponding image of the object onto the eye In this endeavor, accommodation is utilized when an image is made of differing points along the optical axis This process is slow and the time required increases with a person’s age; the ability to accommodate begins to wane after a particular age, to the point where accommodation is no longer feasible For this reason, all the visual surfaces in the cockpit should require as little accommodation as possible, in order to appear on virtually the same point along the optical axis As this is not usually the case in practice (between the primary flight display, glare shield fixture, center console, and heads-up display), the workload of the visual system is increased significantly In addition, normal binocular vision is coupled with accommodation-related convergence movements of the eyes: If a close object is to be visualized clearly, both vision axes must be brought together This drive for convergence and accommodation are coupled This means that every time accommodation occurs, convergence is evoked If the image on the retina is not sharp, the eye begins to initiate accommodation and, therefore, also convergence This leads to further problems of image recognition The ability to accommodate declines with age, as the near point moves further away (presbyopia) The near point of a 20 year old is about 10 cm, whereas it is Ophthalmology for the Medical Examiner 403 about half a meter for a 50 year old Corrective lenses are particularly important for a presbyopic pilot as they must allow the pilot to meet the regulatory standards for distant vision, and at the same time provide a clear picture of a number of objects inside the cockpit at varying distances from the pilot’s eyes It is not physically-technically possible to produce lenses that allow the presbyopic pilot to focus immediately on any object regardless of distance Because cockpit geometry is complex, the pilot may have to search for that segment in the glasses where the image is seen most clearly In addition to this problem, lateral distortion also leads to further, unavoidable visual deterioration The additional effect that chromatic aberrations have on accommodation is only briefly mentioned, but this is an issue that is becoming increasingly important as modern flight decks employ an increasing number of color applications Colored indicators, located at equal visual distance, require changes in accommodation with visualization of each chosen color With regards to the refractive errors, myopia, hyperopia, and astigmatism, more will be detailed in relation to the regulatory standards and examination techniques, as well as refractive methods of eyeglasses and contact lenses In the past few years, refractive corneal surgery has become widely used to correct the refractive errors that occur due to variations in bulbar length This is done by reshaping the corneal surface and flattening it One method is to make radial cuts into the corneal surface, leading to scarring in the periphery that in turn leads to a bowing-up of tissue there, which then leads to a flattening of the central area The same goal is achieved by using excimer-laser-ablation of the central area of the cornea The removal of tissue flattens the corneal curve, reduces the refractive power, and allows a sharp image on the retina despite the long bulbar length This excimer-laser-ablation can also be performed in the intrastromal tissue To this end, the anterior corneal lamella is excised (Lasik), central corneal material is removed, and the lamella is reinserted Common for these procedures is an increased risk for scar formation and subsequent structural changes within the cornea The first case of keratoconus after such surgeries has been reported 404 R Schwartz and J Draeger It is obvious that, besides questions of optical imaging such as resolution, color vision, contrast, and dark adaptations, other flightspecific issues can influence visual performance The relative drying of the corneal surface due to low air humidity in most commercial aircraft is well known, and can be particularly bothersome to older pilots However, the effect of acceleration as having hydrostatic effects on the eyes can be disregarded in commercial aviation but not always in private flying The duty of the aeromedical ophthalmologist is to utilize particular knowledge regarding the actual performance requirements in the cockpit, the work environment of air traffic controllers, the regulatory standards, and to perform accurate examinations ANATOMY AND PHYSIOLOGY OF THE EYE Introduction The human eye (Fig 1) is one of the most complex organs of the body Next to the kidney, it has the highest metabolic rate Hypoxia impairs all visual functions Besides the parasympathetic and sympathetic nervous system, half of all brain neurons participate in the innervation of the eye Visual performance encompasses many different qualities, such as visual acuity, contrast, dark adaptation, glare sensitivity, spatial vision, color vision, and peripheral vision Only with regular and diligent examinations can vision problems that may endanger aviation safety be recognized early Eye Lids and Tear Ducts The eyelids are muscular soft tissue with extensive safety functions Form and function are such that the eye can be covered entirely by closure of the lids Lid closure occurs reflexly by way of mechanical, optical, and acoustic stimulation An even distribution of tear Ophthalmology for the Medical Examiner 405 Eye muscle Sclera Choroid Lens Retina Cornea Iris Pupil Anterior chamber Optic nerve Posterior chamber Optic disc Ciliary body Macula Vitreous Retinal vessels Figure Cross-section of the human eye secretion occurs through regular blinking about 20–30 times per minute, preventing drying of conjunctival and corneal tissues The edges of the lids contain countless sweat and sebum secreting glands, which have the effect of lubricating the lid margins in order to prevent overflow of tear fluid The inner aspect of the lids is lined with conjunctiva Accessory tear glands reside in the upper and lower folds; they produce serous tear fluid along with that from the main ducts, which are located under the temporal area of the upper eyelid Tear film has three layers The outer lipid layers consist of secretions from the sebum and tear glands; they prevent too rapid evaporation of the tear film The middle watery layer’s function is primarily to cleanse the cornea surface and to facilitate a high quality image The inner mucinous layer comes from the goblet cells of the conjunctiva and the main tear gland; it serves as a stabilizer of the tear film upon the otherwise hydrophilic corneal surface Tear fluid discharges by way of the upper and lower tear ducts From there, the tears flow via the upper and lower duct canals into the lacrimal sac and through the nasal tear duct into the nasal cavity 406 R Schwartz and J Draeger The Conjunctiva The conjunctiva is a thin, transparent vascular mucous membrane layer Its mobility upon the underlying sclera and the tissue redundancy in the lid folds allow free movement of the eye ball in all visual directions Production of bactericidal substances inhibit eye infections significantly The Cornea The cornea is inserted into the mildly curved sclera like a watch crystal Its transparency and uniform curvature is a requirement for good optical imaging With a refractive power of about 43 diopters, it contributes the most to the total refraction of the eye The external corneal surface is composed of multiple non-cornified epithelial layers, which can rapidly regenerate when injured The basal cell layers are joined to the very tough Bowman’s membrane by a thin mucosal membrane An injury to this layer does not regenerate tissue, but forms a scar The corneal stroma, which regenerates slowly owing to lack of vasculature, is made of collagen lamellae The stroma is bound in the anterior chamber by the durable Descemete’s membrane This serves as the basal membrane of the single layered corneal epithelium The pumping action of this non-regenerative endothelium maintains the transparency of the cornea The average diameter of the adult cornea is about 11 mm, and the axial thickness about 500 µm The cornea is sensitive to touch via innervation from the trigeminal nerve; injuries are very painful The Sclera Together with the cornea, the sclera forms the protective, outer layer of the eye, onto which six external eye muscles are attached The sclera is white, opaque, and fibrous, and contains collagen and elastic fibers with a high water content Ophthalmology for the Medical Examiner 407 The Lens The lens of the eye focuses the incoming light onto the retina Allowing for accommodation, the portion of refraction attributed to the lens is in the order of 10–20 diopters The lens is clear and biconvex, with a greater curvature on the posterior surface It lies in the back of the anterior chamber between the posterior aspect of the iris and the vitreum, and is therefore a component of the iris–lens– diaphragm system, which separates the anterior chamber from the posterior Zonular fibers are attached to the equator of the lens and connected to the ciliary body (Fig 2) When the ciliary muscle contracts, the tension on the lens, maintained by the zonular fibers, is reduced, and its intrinsic elasticity brings it to a rounder shape This is called accommodation (Fig 3) Since the elasticity of the lens continuously decreases after birth, there is a continued deterioration of the lens’ power of accommodation Accommodation Normal vision (emmetropia) exists when parallel light rays enter the eye and are focused onto the retina (Fig 4) This is the rule when the axial length of the eye is 24 mm The refractive power increases through accommodation, whereby near objects can be seen clearly (Fig 5) The unit of measurement is a diopter (D), which is the reciprocal of the focal length measured in metres (i.e., 1/m) In the case of emmetropia, an accommodation of D will bring an object 1m at a distance of 20 cm — sharply into focus on the retina (Figs 2, and 5) ( ) Refraction Refraction is determined primarily by the axial length of the eye If the eye is one mm too long or too short, a 3D-discrepancy exists The eye axis generally lengthens until puberty, increasing 408 R Schwartz and J Draeger Figure Figure Relaxed ciliary muscle Contracted ciliary muscle nearsightedness and decreasing farsightedness Loss of lens elasticity results in a loss of accommodation, as does an increase in lens density This creates a higher optical density and refractive index with age, and therefore increases nearsightedness, which is additional to the axial ametropia ... 6/15 6/ 12 6/9.5 6/7.5 6/6 6/4.8 6/3.8 6/3.0 20 /20 0 20 /160 20 / 125 20 /100 20 /80 20 /60 20 /50 20 /40 20 /30 20 /25 20 /20 20 /16 20 / 12. 5 20 /10 MAR logMAR Decimal 10 8.0 6.3 5.0 4.0 3 .2 2.5 2. 0 1.6 1 .25 1.00... 0.6 0.5 0.4 0.3 0 .2 0.1 0.0 −0.1 −0 .2 −0.3 0.10 0.13 0.16 0 .20 0 .25 0. 32 0.40 0.50 0.63 0.80 1.00 1 .25 1.58 2. 00 Many individuals, especially young people, possess a visus of >1 In other words,... Body, and the Choroid Iris and pupil Iritis is the most common form of uveitis, and is most often combined with an inflammation of the ciliary body (cyclitis) Recurrence is frequent and often